The present invention relates to a hold-type display device such as a liquid crystal display device, an organic electro luminance (EL) display and a liquid crystal on silicon (LCOS) display and its driving method, and more particularly to a display device suitable for displaying moving images and its driving method.
Display devices are mainly classified into an impulse type display device and a hold-type display device if the display devices are classified from the viewpoint of moving image display. The impulse type display device is a device like a Brown tube of the type that pixels become bright only during a scanned period and a luminance of each pixel lowers immediately after scanning, whereas the hold-type display device is a device like a liquid crystal display device of the type that the brightness corresponding to display data continues to be held until next scanning.
The characteristics of a hold-type display device reside in that although a good display quality can be obtained for still images, a peripheral area of a moving image blurs, i.e., so-called moving image blurring occurs, and a display quality is lowered. This moving image blurring results from a so-called retina after image by which when a line of sight follows a moving image, the viewer interpolates display images after and before motion into a display image having a luminance of a hold-type. Therefore, even if a response speed of the display device is improved as fast as possible, the moving image blurring cannot be eliminated completely.
As a technique of eliminating moving image blurring of a hold-type display device, U.S. Patent Publication No. 2004/001054 (JP-A-2003-280599) discloses the technique of inserting blanking data (black display data) between consecutive display data (hereinafter abbreviated a black display data insertion method), namely, displaying display data and blanking data in one frame period.
As a technique of inserting black display data, U.S. Patent Publication No. 2004/155847 (JP-A-2004-240317) discloses the following technique. When a desired pixel value is to be written in a pixel of a hold-type display device, an effective write operation is concentrated on a partial period of a frame period. In this case, in order to obtain a desired visual pixel value from the write operation during the partial period, a write value during the partial period is set higher than the desired pixel value, so that write values of pixels during a period other than the partial period become relatively low to thereby obtain visual recognition of a moving image similar to that of an impulse type display device. According to this technique, a frame period is divided into m periods (m is an integer of 2 or larger). By representing m periods as first to m-th periods, a desired pixel value multiplied by m is written in pixels during the first period, and 0 is written during the second to following periods. In this display device, if the pixel value multiplied by m exceeds an allowable display range of the display device, an upper limit value of the range is written in the first period, and an excessive portion not written in the first period is written in pixels during the second period. Similarly, an excessive portion not written during the i-th period (2≦i≦m−1) is sequentially written in pixels during the (i+1)-th period to improve visual recognition of moving images. In this specification, this drive method is defined as frame division drive.
In a liquid crystal display device, moving image blurring occurs also due to a slow response time of liquid crystal elements. In order to solve this liquid crystal response speed problem, U.S. Pat. No. 5,347,294 (JP-A-4-365094) discloses the following drive method. A drive voltage to be supplied to the liquid crystal display panel is changed with a difference between an input image signal one frame before and an input image signal of a present frame. Namely, if a gray-scale of the input image signal of the present frame is higher than a gray-scale of the input image signal one frame before, i.e., if an image of the present frame is brighter than that one frame before, a drive voltage higher than a gray-scale voltage of the input image signal of the present frame is applied to the liquid crystal display panel. On the other hand, if a gray-scale of the input image signal of the present frame is lower than a gray-scale of the input image signal one frame before, i.e., if an image of the present frame is darker than that one frame before, a drive voltage lower than a gray-scale voltage of the input image signal of the present frame is applied to the liquid crystal display panel. In this specification, this drive method is defined as emphasis drive.